Oath of the Hero
by MythMaker258
Summary: At the end of the Fifth Ainsworth Holy Grail War, Shirou Emiya and Miyu are sent to the world of My Hero Academia. Can they survive? And can they keep their secret? Only time will tell.
1. Chapter 1

**Hi! So, this will not be a sequel of my A New Fate fic like the other fics involving a Shirou on my page. This will be an entirely new idea and I hope you enjoy it. I hope you enjoy.**

Miyu Sakatsuki laid on a pedestal, in the middle of a formalcraft circle. She looked up at her big brother with tearful eyes.

"Why… did you come?" she demanded. It didn't make sense.

"I heard it… from those people. That the reason you and Kiritsugu-san took me in… was to use my… power… That I am… just a tool. That in Kiritsugu's place, who couldn't find a way to use me… the Ainsworths would use me… to save the world…"

None of it made sense.

"So then why… did you come…?"

The young girl, the child of god, looked at her older brother, the one who had lied to her and used her. She watched as he closed her eyes, then answering.

"That," Shirou Emiya simply said, "goes without saying. I'm an older brother. So I have to protect my younger sister, don't I?"

Shirou took out seven cards, each with a different picture on it. The seven Class Cards. A miracle brought on by the magecraft of the Ainsworths, the partial summoning of seven Heroic Spirits. Light shone from each card and the magic circle around them began to glow.

Miyu could only gasp in surprise.

"I've been trying to figure it out," Shirou explained. "What to do. It's been one mistake after another with me, so this might be one too. But, for this alone is true, I wish upon the Holy Grail.

"I pray you find a world where you won't have to suffer any longer. I pray that you meet kind people. I pray you find friends you can laugh with. I pray you find a warm, small share of happiness."

With each word, the light of this miracle rose, along with a magical circle made entirely of light. Miyu felt her body floating away as the power inside her brought upon another miracle. She looked down, desperately calling for her beloved Onii-chan.

She watched as he walked away to defend her as she began floating away. She watched as he headed to his death. Because the one he was facing was…

No.

No. Please, god, no.

Not him. Anyone but him.

She saw the Heroic Spirit her brother was facing. Gilgamesh, the first myth and the King of Heroes. Like the other heroes, this one was partially installed, rewriting the host's body to allow the host to use the Heroic Spirit's abilities. And this card owner knew how to use them. Angelica Ainsworth. A resurrected mage of the Ainsworth lineage.

Miyu watched as her brother fought against the enemy, flashes of gold were met with two beautiful blades. She watched as her brother's shields fall and chains restrain his movement. Then she watched in horror as the Angelica brought a giant sword to her brother, only to be shocked with a swirl of snow and just like that, both had disappeared.

Though she couldn't see him anymore, Miyu could still sense her Onii-chan, battling an unbeatable foe. Miyu also knew there would be no escape for him. She wished there was a better way. She wished.

 _Scene Break_

Shirou was probably going to lose. There was no other way to look at it. Once she drew that sword, he knew it was over.

The fight in this world, his world, the Unlimited Blade Works, had started out well enough. Shirou was able to create, summon, and fire his swords at much higher speed than Angelica, who couldn't even manage a single hit. He had closed the distance, almost managing to strike her down. But the magus soon found out it would not be that simple, as Angelica had simply displaced space, blocking his strike and forced him into a freefall hundreds of feet in the air.

"Do not make light of the Ainsworths," Angelica had warned as the magus fell.

Angelica then tried to annihilate Shirou by using Divine Constructs, items forged by the divine and godly. Items like these could not be reproduced on the whim, even with the advanced 'Tracing' Projection magecraft of Shirou Emiya. It was impossible.

Of course that never stopped him before.

The magus countered the Divine Constructs with his own, taking them out of play. The result had caused great fire to this strange, wintery world and the magus, after landing, struggled to his feet.

Angelica walked towards him, summoning the sword that would land the final blow. "Though they may have been cheap imitations, you still managed to project Divine Constructs while continuing to sustain this Reality Marble."

Her praises were genuine. A last kindness to one who was about to die.

"Your very existence is flawed," Angelica continued. "You may prove to be a hindrance to the last legend we have written and put into motion for humanity. Your irksome ability, your strange use of magecraft, and your abhorrent self-destructive conviction…"

She raises her sword and the very world around them begins to shift and turn. Wind howls and power radiates.

"They will be torn to pieces together with your world."

Shirou looked at the sword and froze, his instant analysis of the sword causing his mind to break. His eyes will instantly analyze a sword and copy it here, in his Reality Marble. But this one was different. While he had created the highest tier of weaponry, this sword was something different. Not only was it more divine and ancient than anything he had ever seen, but it was so… foreign. Alien. The sword was completely impossible for any human, lest they be from the past, present or future, to understand.

Shirou took his eyes off of the blade, knowing that more exposure will result in madness.

"Oh, wow…" Shirou says. "I'm honored you'd bring something like that out just for me."

As he speaks, blades begin to rise behind him.

"There's no sword in this world that could stand equal to it.

The blades continue to rise, ascending farther into the heavens.

"So… while it may be a bit rude, I'll have to…"

The blades have formed a wave, a tsunami of steel that darkens this world.

"Pay you back," Shirou yells, "with all the swords I have in stock!"

The wave of blades come, each aimed to annihilate Angelica Ainsworth. Despite the wave of death slowly approaching, Angelica was calm and composed.

"Return to the Beginning," she chanted. "ENUMA ELISH!"

The storm of winds created by the original sword clashed with the infinite swarm of blades. Each blades that collided with the swords was brought to oblivion but there was so many, it was like the greatest of Noble Phantasms was going to be covered by the endless swords. However, as slow as it was, the blast began to advance, the swords only providing a temporary barrier.

"It's futile!" Angelica yelled. "Even if you pile all of your blades together, they are no match for the strongest sword!"

Shirou had a sad look on his face, but still hopeful. "Yeah… you got that right. There are times when just one thing can outweigh everything else."

Shirou could only hold the blast back for a few more minutes, and then it happened.

"ONII-CHAN!"

Shirou and Angelica looked around. There was no questioning it. It was Miyu's voice. But where was she.

Then, in a flash of light Shirou and his world was gone, leaving an uninstalled Angelica in the dark cave. Angelica looked at her self, no golden armor and none of Gilgamesh's personalities affecting her. She looked at where Miyu had once been. No trace of the magic circle.

"What just happened?" she questioned. "The grail disappeared… no, it's more accurate to say… everything related to the Grail War has!"

 _Scene Break_

All Shirou could see was brilliant, ever-changing light. He was lost within a kaleidoscope of color.

"W-where…" Shirou weakly said.

"Don't waste your strength boy," a voice came.

Another flash, this time of white light, came and there stood a man who Shirou had never met before. But his identity was deeply rooted in the Heroic Spirit EMIYA's memories.

"Zelretch," Shirou said, staring at the possessor of the Second True Magic.

"Oh, so you have his memories also," the old vampire said in surprise. "That's interesting. Looks like EMIYA is still integrating himself with you."

"Why are you here?" Shirou growled, trying to get into a combat ready stance. "Where are we?"

Zelretch looks around. "This? This is what you would call, the gap between parallel dimensions."

"The gap?" Shirou repeated. "How did I get here?"

Zelretch laughed. "You didn't really think that the girl known as Child of God would simply let herself be sent away. When you sent her off, she had her own wish, one so strong it basically high jacked your wish."

"Miyu did this?" Shirou said in shocked. To think that his little sister's will was strong enough to overtake his own wish on the Grail.

"Indeed. And now you're being sent to a world where the both of you can be happy," Zelretch replied.

"Can't you stop it?" Shirou asked, curious.

"No, the spell is already in motion," Zelretch explained. "All I'm doing is providing a stop gage so we can talk and I can help a bit."

"Why would you help me?" Shirou asked.

"Let's just say…" Zelretch said, "I'm invested in your future. Or one of them at least. Now I'll take care of that Heroic Spirit invading your body but it will cause some… unusual effects. Also, I'll give you her."

Zelretch puts out his hand and opens his palm, a brilliant light shining. When the light faded, a single object, a pentagram with a six point star, floated in midair.

"Hello, Shirou-sama," the pentagram said. "I am Magical Sapphire."

Shirou stared at the object. "Wait, did you just-"

"Bye!" Zelretch cheerfully waved as he let the stop gage go and the two were sent hurtling to Miyu and the new world.

 **And done. Hope you all like it. Also, as to Sapphire, she's from the Fate/Hollow Ataraxia world. You know how Rin got Ruby in that universe but we never saw Sapphire. Well I'm using that as an excuse to bringing Sapphire in this fic. Anyways, I hope you all enjoyed this and please comment!**


	2. Chapter 2

**Hi, I'm back and here's my next chapter. And if you're wondering why I have so many fics, well it's because I just want to get all my ideas out there. Now onto the comments:**

 **To dark knight discord – Thank you.**

 **To rafaelbosca – I will, but it will be slow. I have all sorts of things happening this year so my chapter output will be a bit slow.**

 **To Yoshiki-909 – I won't give any spoilers. You'll have to keep on reading.**

 **To thebebb – I thought my usual writing style, using one fic to act as the starting point then branching off into multiple separate stories, kinda like how every Fate Stay Night route begins with the same scene then branching off into either the Fate route, Unlimited Blade Works route, and the Heaven's Feel route. If I make another fic that's a sequel to 'A New Fate,' I will be sure to add a summary.**

 **To – I plan on writing on my all my fics. I've just been preoccupied with things.**

 **To Megaolix – I found Sapphire to be a necessity for this. You'll see why. Also, it will give Miyu a 'transformation quirk' so she can fit in more.**

 **Now onto the story.**

It was a bustling night in Musutafu, Japan. There was a dangerous hero fight involving the Symbol of Peace himself and a slime-quirked villain that was possessing a boy with a powerful explosion-type quirk. Everyone was so busy talking about the fight that no one had noticed two shooting stars, both falling into Takoba Municipal Beach Park.

 _Scene Break_

Shirou Emiya opened his eyes, hearing the sound of waves crashing. The boy sat up, to find himself on a beach, which was polluted with garbage.

"Ugh…" Shirou groaned. "What happened…?"

The magus boy quickly sat up, desperately looking around.

"Miyu!" Shirou shouted. "Miyu, where are you?"

He desperately searched around for his sister, who had just moments ago brought him to this world. Shirou gave a sigh of relief when he found Miyu, sleeping softly in a clean area of the beach. He scrambled towards her, ready to wake her. She seemed bigger than usual…. No, that didn't matter right now. He needed to make sure she was okay.

"Miyu!" Shirou shouted, shaking her awake. "Please wake up!"

"N-not so loud, Onii-chan," Miyu moaned, tired. The girl opened her eyes and suddenly she gasped, scrambling back. "O-Onii-chan!?"

Shirou was surprised by her shout, blinking in surprise. "What's wrong? Is it an enemy? Are you hurt?"

"N-no, Onii-chan," Miyu said, blushing. "It's just that… your body… it's…"

Shirou blinked and projected a mirror, staring blankly at his reflection. He couldn't believe it. He was younger, he couldn't be much older than 15, 14 at least.

"So this is what he meant by unusual effects," Shirou grumbled, a bit annoyed about his transformation.

"He?" Miyu asked, tilting her head in confusion.

"I think Shirou-sama is referring to my creator, Kischur Zelretch Schweinorg," a voice came ringing out. The siblings huddled together, looking around for its origin. Shirou prepared to summon his weapons.

"Who said that?" Shirou demanded.

"Me," the voice said, as the ground in front of them rumbled. Suddenly, beams of brilliant light came from the ground and the ground around began to open up. Then something emerged from the ground, something with a dangerously powerful magical aura… a blue wand with a six point star.

Shirou and Miyu stared dumbfounded at the wand, severely unimpressed. Th-this thing looks like a child's toy, something

"Good day," the wand greeted politely. "I am the Kaleidostick Magical Sapphire. It is a pleasure to meet you both, Shirou-sama, Miyu-sama."

"W-what?" Miyu said, still not understanding what was happening.

"I remember you now," Shirou said, remembering his encounter with Zelretch. "You were that Mystic Code Zelretch said I'd need."

The wand bowed, or at least bent to make it look like it was bowing. "Indeed. The creator recruited me to help you in this new world."

"Why would we need help in this new world?" Shirou asked. "Is it dangerous?"

Sapphire sweatdropped. "It's complica-"

Sapphire was interrupted by a loud explosion coming from the distance.

"What was that?" Shirou said, getting into a battle position and summoning his swords, Kanshou and Bakuya.

"Seems like a villain attack…" Sapphire mumbled, facing the explosion's origin.

"A what?"

Sapphire turned back to Shirou. "We should go. Zelretch managed to buy you a temporary home here, an apartment in the city."

Shirou nods. "Let's go. Miyu." He looks at her but finds she wasn't even paying attention. He follows her gaze and smiles.

"That's the ocean, Miyu," Shirou beamed, happy he could fulfill his promise. "Told you I'd show it to you."

"The ocean…" Miyu trailed off, gazing at the lovely sea, which was adorned by the stars of the night sky.

Shirou took her hand, still smiling. "Let's go," he said.

Miyu nods and follows him.

 _Scene Break_

"This is it...?" Shirou asked Sapphire, extremely unimpressed.

The apartment was nothing special. No, that was an understatement. It was complete garbage. It was a small studio apartment, with barely enough room to be called comfortable. The kitchen area was miniscule, and Shirou doubted that even he would be able to make decent meals from it. And the bathroom…. No. It can't even be mentioned.

"The creator has always been a bit of a troll, even to his allies," Sapphire spat, hiding in Miyu's hair to avoid being spotted. The superpowers may be common in this world but who knows what a flying wand suddenly appears. "He's probably watching our reaction and laughing right now."

Shirou groaned and laid down on the bed. "That wouldn't surprise me. So, first Sapphire can you please explain what that explosion was and what you meant by 'villain attack?'"

"I'm curious about that myself," Miyu said.

"You might wish to prepare yourselves," Sapphire said. "Especially you, Shirou-sama, because this world is a world of superheroes."

Shirou blinked sitting up. "What?"

"What do you mean her-" Miyu began asking but was interrupted.

"What?" Shirou asked again.

Sapphire flew back nervously. "Shirou-"

"WHAT?" Shirou yelled. "You have got to be kidding me?"

This had got to be some big joke, some prank from Zelretch. He gave up his ideals of a hero, his passion, choosing to save his sister instead of the world. And now he's suddenly in a world with actual superheroes?! God damn you, Zelretch!

Shirou groans again, slamming his body back into the bed, but then realized that this wasn't the best side to show to Miyu. "Never mind, its fine. I'm over it." He sat back up. "I'm sorry. Sapphire, continue your explanation, please."

Sapphire nodded. "As you wish, Shirou-sama. No mages, phantasmal species, or spirits exist in this universe. However, the supernatural does exist to some extent. There is a mutation in this world that humans call 'Quirks.' Over 80% of the population has acquired these quirks and they have created an increase in both super-powered crime and super-powered crime stopping forces. Eventually, the governments intervened, making 'superhero work' a government position and creating academies to train young heroes."

"So instead of fruitlessly attempting to restrict vigilantes and heroes from attempting to help the public," Shirou concluded, "they put limitations and conditions for it. How clever. How do they train these heroes?"

"Academies all over the world have been created to teach these heroes," Sapphire reported. "The most prominent one is called UA Academy and is right here in Japan."

Shirou nodded in understanding and Miyu merely stayed quiet, a thought going through her head.

"Well that's certainly interesting," Shirou said. "So what's this about a mana anomaly? If no mages exist in this world, what caused it?"

"That would be obvious, Shirou-sama," Sapphire said. "It was caused by your actions."

"The Class Cards," Miyu gasped in realization. "They came with us?"

"How?" Shirou asked in a panic.

"The ritual you performed in your original world had not only transported you and Miyu, but it had taken everything related to your world's 5th Holy Grail War," Sapphire explained. "All Servant Cards used in that war are here, in their own individual Mirror World, an artificial space generated by the card and can only be accessed by either a Mystic Code like myself or an extremely advanced magecraft ritual."

"I see," Shirou said in thought. "That's not good but if there are no mages to use the mana, why is it such a big deal?"

"The disruption in the local ley lines could cause abnormal effects all over the world," the wand explained. "Such manipulations could cause abnormal reactions: strange mutations, natural disasters. Anything is possible."

Shirou sighed. "I understand. So you and I will go collect all eight Class Cards?"

"There are multiple things wrong with that statement," Sapphire said.

Miyu and Shirou looked at the wand curiously.

"What do you mean by that?" the magus asked the wand.

"For one thing," Sapphire said, "one of the Class Cards, the Archer card holding the soul of Heroic Spirit EMIYA, has been fully integrated into Shirou-sama's body."

"What?" Shirou said, looking at his body in surprise. It certainly didn't look like it was installed or integrated with EMIYA or whatever that meant.

"Will Onii-chan be okay?!" Miyu asked worriedly. "What if EMIYA tries to overwrite Onii-chan's soul with his own?"

"That won't happen," Sapphire assured the worried little sister. "My creator, as insane as he is, is adept in downloading memories and powers of alternate selves with perfection. The worse that will happen will cause a few flashbacks, most of which you'll experience in dreams."

"That's a relief," Miyu said, sighing.

"We've got another problem," Shirou stated, bringing the attention of the others, "Because my age has dropped significantly, both Miyu and I are classified as minors. If someone discovers us living here alone without supervision it could end badly for us."

"My creator had also thought of that and gave me something to fix this problem," Sapphire said, a compartment coming out of her top. Inside was a test tube, pictures of weapons, and words written in bright red: Dead Apostle.

Shirou narrowed his eyes and looked at both the test tube and the pictures. "So that's how it is… Damn bloodsucker…."

"What is it, Onii-chan?" Miyu asked, curiously.

"Miyu… Do you know what a Dead Apostle is?" Shirou asked slowly.

"T-there a type of vampire species," Miyu said. "Extremely powerful but they died out in the past."

"At least in our world," Shirou explained. "Well Dead Apostles have extremely powerful abilities: enhanced physical abilities, longevity, even Reality Marbles. But there is a unique ability Dead Apostles have…. Do you know the Dead Apostle Ancestor #7? The Forest of Einnashe?"

Miyu shakes her head.

"There was once a Dead Apostle Ancestor, basically a high-ranking Dead Apostle, named Einnashe, who transformed into one of the Dead via magecraft," Shirou told her. "He was slain ages ago by a certain vampire but that's another story. Anyways, when Einnashe died, his body was left under a bloodsucking tree, which quickly absorbed his blood, turning both the tree and even the entire forest into a Dead Apostle Ancestor."

"What does that have to do with anything," Miyu asked politely.

Shirou merely shows her the picture of one of the weapons, a giant black axe with a red handle and strange red designs that caused the axe to look as if it's covered in blood. "This is the Noble Phantasm Half-dead Blood Axe: Blood-sipping Bestial Axe," Shirou explained. "It's a unique axe that was owned by the Viking Eric Bloodaxe and was created using the remains of a demonic beast. The demonic beast still lives as the axe, however, and gorges on the blood of its victims."

"But what does that have to…" Miyu stops in her tracks. "D-do you mean… Did EMIYA ever hunt Dead Apostles with that axe? Did he create a weapon with the powers of a Dead Apostle?"

Shirou nods. "During his time with Alaya, EMIYA had slain countless monsters with various weapons. However, I doubt he remembered them. Frankly, there are too many weapons in here to make sense of. I can sense a few of these axes that have tasted the blood of a Dead Apostle, but I wouldn't have even thought of this if it weren't for Zelretch. Guess the bloodsucking troll is good for something after all."

"But what will that get you?" Sapphire inquired, extremely curious. "You can't just swing an axe and expect it to fix your problems."

"That's where this comes in," Shirou shows them the picture of a sword, no it was similar to a ceremonial dagger.

"What's this?" Miyu asked.

"Oh…" Sapphire gasped. "The Sword of Paracelsus, the origin of all Azoth Swords. If my creator's unique sword is the final form of an Azoth Sword, this is the prototype. It has the ability to increase magecraft functions and release instant long-term rituals. But it also has a second feature…. Shirou-sama. You wouldn't!"

"Wouldn't what?" Miyu asked, growing concerned by the urgent tone Sapphire used.

"The Sword of Paracelsus's main feature is to act as a sort of 'quantum computer' that analyzes and responds to magic properties of its target, then encroaching them and making it its own. It's a magic power stealing weapon," Shirou explained.

Miyu was silent. "O-Onii-chan… You plan on becoming undead?"

Shirou grew alarmed. "Of course not! That's too idiotic, even for me! I just plan on stealing one particular power the Dead Apostles have! Their eyes!"

Miyu blinked and Sapphire was stunned. "Their eyes?" the two asked.

Shirou sighed. "Vampires have a unique Mystic Eyes that are categorized as Gold under the Noble Colors system, the Mystic Eyes of Enchantment. They allow high levels of hypnosis to be used. Humans can have them too, I've heard of a few users to exist."

Miyu and Sapphire looked relieved at the news. But then Sapphire looked at the test tube.

"What's that then?" Sapphire pointed at the test tube.

"I honestly don't know," Shirou said, picking it up. "Looks like mercury…."

He opened it and instantly the mercury expanded, causing Shirou to drop the flask on the floor. The mercury continued to grow, but parts of it began to change. Its pigment changed, causing parts of it to resemble clothes and its shape became humanoid. In no time at all, they were staring at what seemed to be a maid golem, completely made out of mercury.

"Good evening," the golem greeted. "I am Trimmau, the former maid and bodyguard to Lady Reines El-Melloi Archisorte, heir to the El-Melloi family. Lord Zelretch had acquired me from Lady Reines and her associate, Lord El-Melloi II, to have me assist you all on your journey, Lady Miyu, Lady Sapphire, Lord Shirou."

The three all stared at the maid in silence. That is until Shirou spoke his first word.

"What?" he said.

 _Scene Break_

Momo Yaoyorozu was sitting in her room, preparing for the recommendation tests for UA academy. While she probably didn't need to, to her studying was essential. She honestly couldn't wait to go to UA. Her family's connections had easily gotten her recommendations, not that she needed them. Praised as a genius and equipped with a highly versatile quirk, Momo knew she could easily pass UA's regular admission exams. The only reason why she didn't was because her mother thought it was beneath her.

"Momo!" Speak of the Devil. "Please come down! There is someone I'd like you to meet!"

Momo reluctantly put down her book and headed downstairs to the Great Hall of her house. Oh, did I say house? I meant mansion. Inside the Great Hall sat her mother, but what truly surprised her were the people standing next to her. One was a maid, obviously, who seemed to have a mutation quirk as her body was entirely made of some metallic substance. The second was a young girl, 10 or 11. She had pitch black hair and fair skin, but had a stoic expression on her face, one that shouldn't be on a child. The last one was a red headed boy, her age, good looking and had an athletic build. Not that she noticed or anything.

"Momo," her mother said, "I'd like you to meet our new maid, Emiya Trimmau. Trimmau recently applied to work on the condition she get a good enough pay and that we provide house and board. I agreed to her terms. She and her niece and nephew are going to stay in the servant quarters. I think you'll like them. Little Miyu-chan even has the most adorable quirk… What was it, dear?"

"M-magical girl," Miyu said with a straight face but Momo suspected she was a tad embarrassed by the fact. Momo also wondered what type of quirk something like 'magical girl' would be.

"That's it," her mother said unusually cheerfully. "Now I must be going, I have much work to do. Please get to know her children, Momo. They'll be staying here for a while."

And with that her mother left, leaving her along with these strangers.

"It is a pleasure to meet you, La- I mean Yaoyoruzu-sama," Trimmau bowed.

"Hello, Trimmau-san, nice to meet you," Momo greeted, then turning towards Miyu. "And nice to meet you too, Miyu-chan!"

Miyu kept her stoic expression. "Lovely to meet you to, Yaoyoruzu-san."

The redheaded boy smiled. "It's nice to meet you, Yaoyoruzu-san."

Momo looked at him and he stretched out his hand in friendship. "My name is Emiya Shirou, nice to meet you!"

 **And done with that chapter! Honestly I think I did a decent job but please send a comment if you have something to say. Alright, bye!**


End file.
